This invention relates to electrical contact switches ordinarily prone to failure due to erosion that develops as a result of the occurrence of electrical arcs between the respective surfaces of the contacts. More particularly, this invention relates to electrical contact switches provided with protection against such erosion.
Most contacts that are used in electrical switchgear are prone to erosion due to electrical arcs that are developed at the make and break of a circuit. Arcing is one of the major problems that can cause catastrophic failure of switchgear.
Many attempts have been made to reduce or eliminate the arcing and hence the erosion. For example, U.S. Pats. Nos. 4,345,130 and 3,154,660 disclose controlling of arc erosion of the contact surface through the use of specific materials to form the contact. In U.S. Pat. No. 4,345,130, an interrupting layer of Cu-W alloy is bonded to a base plate of an electro-conductive material, and a contacting layer of Ag-WC alloy is bonded to the Cu-W layer. The Ag-WC alloy layer is said to provide a low and stable contact resistance, while the Cu-W alloy layer is said to provide arc erosion resistivity and anti-weld capability. In U.S. Pat. No. 3,154,660, a copper-cadmium layer formed on a copper contact is said to limit the effects of arc erosion. Such approaches, however, require the use of specific alloy materials to control arc erosion.
Arc extinguishing elements to control arc currents that may develop between the contact surfaces of breaker elements are disclosed in U.S. Pat. Nos. 4,409,444 (arc shields surrounding the contact and arc runways that adjoin the contact) and 3,402,273 (arc chambers which closely surround cooperating contacts of a circuit breaker to hasten extinguishment). Such means, however, attempt to control the arc only after it is formed.
U.S. Pat. No. 3,636,292 discloses a vacuum switch comprising two pairs of operating contacts arranged in a vacuum chamber and operable by a drive means. A semiconductor diode, mounted in the vacuum chamber, bridges one of the two pairs of operating contacts. Interruption of the alternating current circuit opens the pair of operating contacts bridged by the diode prior to opening of the other pair of operating contacts. An elaborate contact drive means comprises a control means that actuates the contacts only when the alternating voltage at the diode is poled for forward direction. As a result of the use of the diode, interruption in the flow of alternating current occurs with formation of a nominal arc, and contact temperature and evaporation are reduced. However, the switch disclosed in the patent is limited for use in a vacuum chamber.
Various semiconductor switches are also known. For example, U.S. Pat. No. 4,365,170 discloses a semiconductor switch comprising a PNPN switch having a semiconductor PNPN structure with at least three PN junctions. Other examples include U.S. Pat. Nos. 4,380,021, 3,784,886 and 3,427,512. However, such semiconductor switches are not employed in switchgear of the type discussed above, wherein arc erosion of contacts is a common problem.
Further, it is well-known that semiconduction devices when used in electrical circuits allow leakage of currents between the supply and load (source/drain).
Accordingly, the present invention provides an electromechanical switch which evidences minimal contact erosion due to arcing. The present invention provides an electrical contact capable of making and breaking electrical contact without substantial erosion of the contact surface and, additionally, provides an electrical contact in which current is permitted to flow only when the contact gap has been completely closed and in which current is interrupted prior to opening the contacts. Furthermore, the present invention minimizes I.sup.2 R losses of the contact.